The long term objective of this project is to enhance our understanding of the physiological regulation of cholesterol biosynthesis and degradation. Regulation of these processes play major roles in achieving cholesterol homeostasis. Elevated serum cholesterol is a major risk factor in the development of atherosclerotic vascular diseases. The regulation of hepatic HMG-CoA reductase and cholesterol 7alpha hydroxylase gene expression by hormones and dietary factors will be investigated using various rat models. These two enzymes catalyze the rate-limiting reactions of cholesterol and bile acid synthesis. The basis for the rapid increase in cholesterol 7alpha hydroxylase mRNA levels in response to thyroid hormone will be investigated. Rates of transcription will be determined. The promoter region of the cholesterol 7alpha hydroxylase gene will be analyzed to determine whether putative thyroid responsive elements actually function as such. The effects of other hormones such as insulin, glucagon and glucocorticoids on cholesterol 7alpha hydroxylase mRNA levels will be examined. Enzyme activity and protein mass will be measured to determine whether changes in mRNA levels fully account for changes in cholesterol 7alpha hydroxylase activity. The effects of cholesterol and bile acid feeding on rates of transcription will be determined. In terms of HMG-CoA reductase, the possible effects of insulin and glucagon on transcription will be determined. The mechanism(s) by which thyroid hormone and glucocorticoids affect HMG-CoA reductase mRNA stability will be studied. The basis for the marked decrease in HMG-CoA reductase activity without a significant change in mRNA levels that occurs in response to cholesterol feeding will be examined. Possible effects on reductase protein levels and activation/inactivation of the enzyme by phosphorylation or thiol modification will be investigated. Certain other enzymes in the cholesterol biosynthetic pathway such as HMG-CoA synthetase, farnesyl pyrophosphate synthetase and mevalonate kinase which are felt to be coordinately regulated with HMG-CoA reductase will be investigated. The degree of similarity of mechanisms of hormonal and dietary regulation of these enzymes will be determined.